1. Field of the Invention
The present invention generally relates to light receiving devices, and particularly relates to a device for receiving an optical pulse signal and converting it into a voltage pulse signal.
2. Description of the Prior Art
The optical transmission technique is widely used for transmitting signals, and there is a high demand for inexpensive optical transmission devices having a large transmission capacity. A light transmitter on the transmission side sends optical pulse signals after converting voltage pulse signals into the optical pulse signals. A light receiver on the receiving side receives the optical pulse signals, and converts them into voltage pulse signals.
FIG. 1 is a circuit diagram of an example of a related-art optical transmission device. The optical transmission device of FIG.1 includes a transmitter and a receiver. The transmitter includes a transmitting unit 11 and a LED (light emission diode) 12. The receiver includes a PD (photo diode) 13, a pre-amplifier 14, an amplifier 15, and a comparator 16.
FIGS. 2A through 2E are time charts showing wave forms of signals observed at various points of FIG. 1. Provided with an input voltage signal as shown in FIG. 2A, the transmitting unit 11 applies a current as shown in FIG. 2B to the LED 12. The LED 12 emits light to send an optical signal, which is then received by the PD 13. At this time, a current as shown in FIG. 2C is generated through the PD 13. The pre-amplifier 14, which is an I-V converter to convert a current into a voltage signal, generates an output voltage signal as shown in FIG. 2D. The voltage signal of FIG. 2D is then amplified by the amplifier 15 before the comparator 16 compares the amplified signal with an appropriate threshold value Vth. As a result, a voltage pulse signal as shown in FIG. 2E is obtained.
As shown in FIG. 2D, the output signal of the pre-amplifier 14 is not precisely a pulse signal because of the resistor and the capacitor included in the PD 13. In order to explain this, a simple model circuit of the pre-amplifier 14 and the PD 13 connected thereto is shown in FIG. 3A with the capacitor of the PD 13 separately shown. The model circuit of the pre-amplifier 14 in this figure is a simplified circuit only to explain a basic principle of the pre-amplifier.
In FIG. 3A, the pre-amplifier 14 includes a resistor 21 and a Schottky diode 22, and the PD 13 includes a PD 23 and a capacitor 24. Here, Vcc denotes a power voltage. When an optical signal as shown in FIG. 3B is received by the PD 23, a current commensurate with this optical signal would flow through the PD 23 if there were no capacitor 24. However, the existence of the capacitor 24 delays a rise and a fall of the voltage appearing at the output node of the pre-amplifier 14. As a result, a voltage shown by a solid line in FIG. 3C is obtained at an output of the pre-amplifier 14.
A signal shown by a dashed line in FIG. 3C is a voltage which would appear at the output node of the pre-amplifier 14 if there were no Schottky diode 22 in FIG. 3A. In reality, the Schottky diode 22 of the pre-amplifier 14 clamps the output voltage, thereby providing the voltage signal shown by the solid line in FIG. 3C.
Because of the reason described above, the output signal of the pre-amplifier 14 shown in FIG. 2D does not properly reflect the transmitted signal. Therefore, the output signal of the comparator 16 (FIG. 2E) obtained through a comparison with a threshold value has a pulse length longer than that of the transmitted signal. Even if the signal shown in FIG. 2D is deformed, there is little problem when the pulse length of the transmitted signal is relatively long, i.e., when T is far greater than Te. However, when the pulse length is shortened such that T/2 is less than Te, a difference in the pulse length between the transmitter side and the receiver side becomes undesirably large. In such a case, a problem arises when a modulation scheme representing 0.5 and 1.5 using the pulse length is employed.
FIG. 4 is an enlarged view of the output signal of the pre-amplifier 14 shown in FIG. 2D. In the voltage signal form of FIG. 4, the effect of the Te portion can be minimized if the threshold voltage can be set to a voltage Vr slightly lower than the maximum voltage Vh. Namely, even if the pulse length is shortened on the transmitter side, a voltage pulse signal having a similar pulse length can be obtained on the receiver side.
Although it is preferable to use a threshold voltage slightly lower than the maximum voltage Vh, it is very difficult to set such a threshold voltage due to voltage fluctuation or the like. The above description has been provided, for the sake of simplicity of explanation, by considering a threshold value for the output signal of the pre-amplifier 14. In reality, however, the amplifier 15 amplifies the signal before the comparator 16 compares it with the threshold value.
A photo diode used as the PD 13 ranges from an expensive one having a capacity of an order of picofarads to an inexpensive one having a capacity of an order of several tens of picofarads. When a pulse frequency of a transmitted optical signal becomes several tens of megahertz or higher, the expensive photo diode having a capacity of an order of picofarads should be used, resulting in higher prices of the optical transmission devices. This is not desirable for optical transmission devices which are mass-manufactured and used in the general-purpose computer communication.
Accordingly, there is a need for a light receiver which can reconstruct a voltage signal having a pulse length similar to that on the transmitter side in order to achieve a high-speed optical transmission.